Thermoplastic materials have been known for many years. Such thermoplastic materials as polyethylene and polypropylene are commonly used in injection molding of useful articles and are commonly extruded into strands or filaments, in thin sections into film, and in much thicker sections into tubing and pipe.
Such tubing and pipe, and many types of plastic articles, must be connected to one another in order to be useful. For the most part, such connections have been accomplished through the use of mechanical pressure type devices. U.S. Pat. Nos. 3,195,933; 3,501,177 and 3,977,708 show a number of such mechanical and pressure type devices incorporating threads and tubing deforming devices applying substantial pressure. These are typical of the connections used between tubing or pipes and between plastic pipes and plastic valves or other fittings; also see U.S. Pat. Nos. 4,185,807 and 4,343,456 for plastic valves to be connected to plastic pipes.
Such mechanical and pressure type connections are used on all plastics, including polyethylene and polypropylene, and also including perfluoroalkoxy (PFA) fluorocarbon resin known by its trademark Teflon, of Du Pont, Wilmington, Del.
Connections between thermoplastic articles (including tubes and pipes) have also been created by certain types of welding. For instance, spin welding is accomplished by rapidly spinning one plastic part relative to another, thereby creating heating to fusion and resulting in welding the mating faces of the articles together. Some welding has been accomplished of certain plastics by the use of solvents which soften the joint area of the plastic article which may be inserted into or may embrace similar plastic parts, and will be welded thereto as the solvent evaporates and the plastic is again hardened or cured.
Welding of thermoplastic pipes together has been accomplished, as disclosed in U.S. Pat. No. 3,013,925, by inserting a heated platen between the ends of lined pipes and causing the pipes to bear against the heated platen to fuse the plastic after which, the platen is removed and the pipes are pressed together under pressure until welding has occurred and the thermoplastic material has hardened again as it cools. Other patents, such as U.S. Pat. Nos. 3,729,360: 4,263,084; 3,727,289; and 3,616,024 illustrate this same welding process.
Similarly, welding of pipes and solid plastic articles is shown to have been effected in U.S. Pat. No. 3,276,941 by contacting the ends of the articles or pipe by a heated element; and in U.S. Pat. No. 3,998,682, peripheral heaters embrace the peripherals of a pipe and connector fitting made of two different types of plastic; and U.S. Pat. No. 3,117,903 discloses the heating of the ends of pipes by inserting them into a hot glycerine bath prior to end to end assembly where welding occurs.
In U.S. Pat. Nos. 3,404,051 and 3,968,195, an open flame is used for fusinq nylon rope and for melting a closure disc respectively to accommodate welding. In U.S. Pat. No. 3,968,195, the open flame actually embraces the closure disc to be melted and the ends of the thermoplastic tube, which is to be welded to a similar plastic tube, also in the open flame. Of course, the products of combustion may collect on the plastic.
In U.S. Pat. No. 3,796,625, there are disclosures of the use of radiant heat used to weld the edges of plastic film together In U.S. Pat. No. 3,383,267, it is suggested that the radiant heat welding can be used to weld films of Teflon PTFE and FEP. However, it is known that Teflon PTFE will not bond to itself.
Radiant heat is also employed in U.S. Pat. No. 2,665,738 to heat the edges of plastic workpieces, the surfaces of which are softened by the heat radiating from a heating element without actually touching the heating element to the plastic. In no way does this relate to making leak-proof, secure joints.
Plastics made of fluoropolymers of various types have only recently been used in the manufacture of fittings for pipes and tubing of the same material. Such fittings include valve housings, tees, elbows. Such fluoropolymers are different than earlier thermoplastics, such as polyethylene and polypropylene, in that they withstand service temperatures in the range of 300.degree. to 500.degree. F. in contrast to the service temperatures of less than 175.degree. F. for components of most elastomers and plastics.
There has been no butt welding of plastic components or ducts or pipes made of fluorocarbons which are also melt processible thermoplastics and particularly, perfluoroalkoxy, known as Teflon PFA, or of fluoroethylenepolymer, known as Teflon FEP, or of polytetrafluoroethylene, known as Teflon PTFE.
Early fluorocarbon resins were developed in 1938, and were known as TFE or PTFE, more specifically known as polytetrafluoroethylene. Although this material was loosely referred to as a thermoplastic, it did not act like other thermoplastics. PTFE, when melted, does not flow like other thermoplastics and it cannot be injection molded. Rather, it must be shaped initially by techniques similar to powder metallurgy. PTFE will not bond to itself in any welding process. Important attributes of PTFE are that it has a high continuous service temperature, approximately 260.degree. C. or 500.degree. F., and has a high degree of chemical inertness.
The next improved fluorocarbon resin was put on the market in 1960, and was known as FEP, or fluoroethylenepolymer. This material is melt processible, and therefore may be injection molded and extruded by melt processing. However, molding of FEP is extremely difficult and requires that the molds be of special design. FEP is extremely difficult to injection mold, and no commercially sold pipe fittings or valves were made of it. This material is known for its high degree of chemical inertness and for its high continuous service temperature of 205.degree. C., 400.degree. F.
Subsequently, an improved material known as PFA or perfluoroalkoxy was brought to market in 1972. This new material is melt processible and may be more easily injection molded and extruded. Pipes and tubing of this material have been found useful because of the high degree of chemical inertness. However, fittings and appliances have been joined only by threaded or pressure type joints. The surface of the material when molded is extremely smooth. Adhesive bonding of PFA articles is widely used, but only after the surface is chemically etched and anhydrous solutions of elemental sodium which removes fluorine from the polymer molecule. PFA is widely used because of its high degree of chemical inertness, low absorption, and high continuous service temperature of 260.degree. C., 500.degree. F. as linings for pipes and ducts and pumps.